1. Field of the Invention
The present invention pertains to the field of heat dissipation. More particularly, the present invention pertains to heat pipes that have pumping capabilities and the use thereof in cooling computers or other devices.
2. Description of Related Art
Attaching a heat pipe to an electronic component such as an integrated circuit is a known and successful technique of moving heat away from that electronic component. Unfortunately, continued efficient use of heat pipes in this manner may be jeopardized by the increasing heat generation per unit area of electronic devices. A technique allowing a heat pipe to accommodate larger amounts of heat per unit area may advantageously allow the continuing use of heat pipes to remove heat from electronic components. Additionally, a technique allowing heat pipes to more easily overcome gravitational forces may allow longer heat pipes and/or new applications involving vertical displacement from one end of a heat pipe to the other.
In the conventional heat pipe, one end of the heat pipe is exposed to the heat source and the other end of the heat pipe is exposed to the heat sink, which is at a lower temperature than the heat source. Heat is absorbed from the heat source by evaporation of a liquid-phase working fluid to vapor phase inside the heat pipe at the end exposed to the heat source (the evaporator). The working fluid in vapor phase with its absorbed heat load is thermodynamically driven to the other end of the heat pipe due to a pressure difference created between the heat source and the heat sink.
The heat load is rejected by the working fluid to the heat sink, with consequent condensation of the working fluid to liquid phase at the heat sink end of the heat pipe (the condenser). Then, without leaving the same heat pipe chamber, the condensed working fluid is returned in liquid phase to the heat source end of the heat pipe by a capillary structure located inside the heat pipe.
The capillary structure is typically an elongated wick structure extending for substantially the full interior length of the heat pipe. Capillary flow is the flow of the fluid on or through the wick structure. The capillary pumping capability of a heat pipe is determined in part by the extent to which capillary forces acting on the liquid-phase working fluid in the pores of the wick structure inside the heat pipe dominate over the gravitational force acting on the liquid-phase working fluid.
As electronic devices and especially integrated circuits continue to consume significantly more power while maintaining approximately the same size, the heat generated per unit area (the heat flux) rises. The increasing heat flux may be problematic in causing a "dry out" phenomenon in the heat pipe. The "dry out" phenomenon is produced when the same amount of liquid corresponding to the evaporated amount of liquid in the evaporator is not supplied to the evaporator by the capillary action. If insufficient liquid is supplied, the heat transport efficiency may be adversely affected since there is insufficient working fluid to transport heat from the evaporator.
Some prior art approaches to improving the heat carrying capacity of heat pipes include the use of pumps to replace or supplement capillary forces as the mechanism to return the working liquid to the evaporator. For example, the approaches described in U.S. Pat. Nos. 4,898,231 and 4,470,450 utilize a separate liquid phase chamber and a liquid phase pump to return liquid to the evaporator. While such systems may be appropriate in extra-terrestrial application requiring large amounts of heat to be transported over long distances, these approaches may not be appropriate for relatively compact devices such as portable computer systems.
Thus, the prior art may not provide an adequate pump-assisted heat pipe that may be used in portable computers or other devices. Additionally, the prior art may not provide a solution which enhances capillary flow within a single, shared vapor and fluid flow chamber of a heat pipe.